The IMAGE_ARG_* mechanism doesn't work with multi-arch builds

[imsodin]

I am successfully using the multi-arch capabilities to build images for both amd64 and arm64, however not when using an image obtained through registry-image with IMAGE_ARG_* as introduced in #52 (we need it as the image is in ECR/private). It apparently uses the host arch for the base image regardless of target arch.

Small reproducer. For good measure I use oci-build-image from master tag, currently 1aac28e to get the latest buildkit updates - same result with v0.11.0:

Dockerfile:

ARG from_image=golang:1.19.5-alpine3.16
FROM ${from_image}

RUN echo test

Pipeline:

resource_types:
  - name: registry-image
    type: registry-image
    source:
      repository: "concourse/registry-image-resource"
      tag: 1.7.1

resources:
  - name: repo
    type: git
    icon: github-circle
    webhook_token: ((github_webhook.token))
    source:
      uri: https://github.com/nsat/maritime-go-bootstrap.git
      username: ((github.github_bot_username))
      password: ((github.github_bot_password))
      branch: MARITIME-1334-docker-from-image
      paths:
        - build/test
        - pipeline/task/build-docker-dualarch.yaml

  - name: from-image
    type: registry-image
    icon: docker
    source:
      repository: golang
      tag: 1.19.3-alpine3.16

jobs:
  - name: release-image
    plan:
      - in_parallel:
        - get: repo
          trigger: true
        - get: from-image
          params:
            format: oci

      - task: docker-build
        privileged: true
        config:
          platform: linux
          image_resource:
            type: registry-image
            source:
              repository: concourse/oci-build-task
              tag: master
          inputs:
            - name: repo
            - name: from-image
              optional: true
          outputs:
            - name: image
          caches:
            - path: cache
          params:
            CONTEXT: repo/build/test
            IMAGE_ARG_from_image: from-image/image.tar
            IMAGE_PLATFORM: linux/amd64,linux/arm64
            OUTPUT_OCI: true
          run:
            path: build

Relevant output:

#9 [linux/arm64 2/2] RUN echo test

#0 0.084 .buildkit_qemu_emulator: /bin/sh: Invalid ELF image for this architecture

#9 ERROR: process "/dev/.buildkit_qemu_emulator /bin/sh -c echo test" did not complete successfully: exit code: 255

------

 > [linux/arm64 2/2] RUN echo test:

#0 0.084 .buildkit_qemu_emulator: /bin/sh: Invalid ELF image for this architecture

If I remove the parts about IMAGE_ARG_*, it just works.

Tagging @vito as the author of #52 and @rdrgmnzs as the author of multi-arch support (567071e).